@Lokes Depends how close and what kind of concept. Directly tracing, copying, or editing another user's art is going to be in violation of the following rule if someone reports it or I catch it (there's no way I will remember every item I approve, there are way too many):
IMPORTANT: Copyrighted and stolen material, submitting an item that contains copyrighted designs/content (eg. fanart of another media and so on), or stolen/traced art may lead to the immediate suspension of your ability to submit custom item designs.
Items added through the custom content feature will become an official asset owned by the site and, to prevent any infringement on the copyright of others, it is extremely important that all pieces are original works and do not copy from others.
This means you should absolutely never submit anything that is:
A direct reference to another game, show, character, or any other creative work or piece of media that is not in the public domain.
Trademarked symbols or names from companies and organizations.
Traced art or art that is heavily referenced from another artist's work.
Your own art that has been published elsewhere by another company that owns the exclusive rights to use it.
Having a similar idea though definitely does not fall under this, especially for simple concepts. When people are making items loosely based on site assets (like adding to an existing set), this is even more likely to occur.
Essentially: It's not about the concept itself, it's about the execution and specific design/appearance of the concept.